1. Field of the Invention
This invention relates generally to the field of semiconductor equipment and more particularly concerns optimal methods and apparatus for processing wafers with use of downstream plasma and gas injection.
2. Description of the Related Art
Downstream plasma processing systems are often used to process semiconductor wafers or substrates for fabrication of integrated circuits. For example, downstream plasma processes are commonly used in etching, photoresist removal, chemical vapor deposition (CVD), etc. The downstream plasma processes are typically carried out by systems that generally include a plasma processing chamber to provide a controlled setting.
The high temperatures produced within the plasma provide a source of convective heat transfer to the processing wafer. Subsequent heating of the wafer can produce wafer temperatures high enough to produce thermal damage to critical structures on the wafer, or detrimentally impacting the process, thus reducing device yield or requiring extensive further processing to remove the damage. Prior art equipment attempts to solve this problem by increasing the distance from the plasma source to the wafer using paths extended beyond ten inches.
The presence of electrons and ions that remain in the effluent gases which interact with the wafer surface can activate a mobile charge (Qm) on the surface of the wafer or induce damage to charge damage to underlying gate. Either of these effects may damage or destroy some or all of a device's structures before the it's fabrication can be completed. Long paths are sometimes used to minimize this effect.
Downstream injection has been used to improve downstream plasma processes by creating specific reactants which are capable of highly selective reactions. Specific examples are the addition of Cl2 to a the effluent of a fluorine containing plasma (e.g. NF3) to form ClF. The ClF is a highly selective etchant used for the highly selective removal of silicon nitride in the presence of silicon oxides. (J. Electrochem. Soc., (1989), 136, 2032) Most notably used for the removal of oxide isolation masks in LOCOS and PBLOCOS as well as in shallow trench isolation (STI) process flows.
A second example is removal of native silicon dioxide from silicon wafer surfaces prior to diffusion processes. (Jpn. J. Appl. Phys., (1994) 33, 2207.) In this case NF3 is added downstream of Hydrogen plasma.
FIG. 1 shows one example of a downstream plasma source wafer processing apparatus 10. In this example, the apparatus 10 includes a microwave cavity 16 that is supplied with microwaves from a microwave supply source 18. The microwave cavity 16 is coupled with a tube 12 which has an input 14 where gas is typically injected. In such a configuration, gases such as hydrogen are generally injected into the tube 12 through the input 14. The gases are then energized to form downstream plasma by the microwaves in the microwave cavity 16. An input 22 leads into the tube 12. As the downstream plasma flows freely down the tube 12, NF3 may be injected into the tube 12. During this lengthy straight-line travel without obstruction or interruption, the downstream plasma may become less reactive through recombination. Then the downstream plasma may serve to process a wafer 20 near an opposite end of the tube 12. Unfortunately, such a structure is generally large and the tube 12 is very long because the downstream plasma typically needs a long paths of travel distance for recombination to occur. Therefore, such structures can make wafer processing equipment large and space consuming. Unfortunately, in such a structure, it can be difficult to control and manage the types of downstream plasma species produced during the downstream plasma production and transportation process.
Another embodiment of a downstream plasma processing apparatus is shown by U.S. Pat. No. 5,814,154 entitled “Short-Coupled Path Extender for Plasma Source.” This U.S. patent is hereby incorporated by reference.
If the types of species are not controlled properly, wafers may become damaged due to unforeseen reactions taking place on the wafer. In view of the above, an apparatus that utilizes gas injection in strategic locations to enable control of plasma species produced so wafer processing can be more uniform and controlled.